1. Field of the Invention
The present invention relates to a voltage dividing resistor and a voltage dividing circuit. In particular, it relates to a voltage dividing resistor and a voltage dividing circuit which may be used in a reference voltage divider of an A/D converter.
2. Description of the Related Art
In an A/D converter, a voltage dividing circuit 30 as shown in the left side of FIG. 8 has heretofore been used to generate a group of voltages as comparative references.
In the voltage dividing circuit 30, a reference voltage 32 is divided by a voltage dividing resistor 33 provided with taps to generate comparative reference voltages at a group of necessary output terminals 37. The generated comparative reference voltages are supplied to comparators 31 and compared with an input signal from a comparison input terminal 34, and the input signal is converted into digital data.
Each of impedances of the output terminals 37 arranged as described above has a combined impedance of the resistance between the output terminal 37 and an input terminal 35 and the resistance between the output terminal 37 and an earth terminal 36 in parallel (It is noted that the voltage 32 in FIG. 8 may be regarded as a short, because the discussion is given here on high frequencies.). Accordingly, the impedance at the output terminal 37 at the 1/2 position of the resistance element is highest as shown in FIG. 5. When the A/D converter is in action, various noise currents flow into the output terminals 37 from the comparison input terminal 34 of the subsequent comparators 31 through a parasitic capacity of the comparators 31 or the like to cause fluctuations in voltage. Magnitude of the fluctuations is proportional to the impedance of the output terminal 37 and frequency of the signal. Accordingly, the fluctuation is largest at the output terminal 37 at the 1/2 position of the resistance element. Further, voltages per se, which are to serve as references in A/D conversion become more unstable as the frequency becomes higher. This creates a factor which adversely affects the effective dividing capacity of the A/D converter and creates distortion at a high frequency.
Japanese Unexamined Patent Publication No. 22137/1993 discloses an A/D converter which has wire connections improved in such a manner that a wiring for an input signal is led to a plurality of comparators so as to improve high frequency characteristics of the A/D converter. In contrast thereto, the present invention is directed to improvement of high frequency characteristics by stabilizing comparative reference voltages.
To solve the above problem, it has heretofore been practiced that a capacitive element 38 is connected to the output terminal 37 at the 1/2 position of the resistance element whose impedance is highest, thereby lowering the impedance in a high frequency range. In other words, the capacitive element 38 is connected to the output terminal 37 at the middle of the resistance element with a view to lowering the impedance in a high frequency range. FIG. 9 is a sectional view showing one form of the structure of the voltage dividing resistor 33 used in the circuit in FIG. 8. A silicon oxide layer 41 as insulator is provided on resistor 42 prepared by doping a p-type silicon substrate with n-type impurities. The silicon oxide layer 41 is formed with holes, and aluminum electrodes 43 are disposed therein to provide output electrodes.
Since the influx of noise currents occurs mainly through the parasitic capacity, magnitude of the noise currents is large in a high frequency range, i.e., the noise expends to a high frequency range. Accordingly, the connection of the capacitive element is an effective means to suppress fluctuations of the voltage.
However, the above-described means has a problem, in that it is necessary to allocate a large space in a chip to the capacitive element for eliminating influence of the noises, thereby leading to an increased cost of a chip. Further, the capacitive element is connected to the resistance element in a localized manner, i.e., at the one point in the middle of the resistance element. Accordingly, as shown in FIG. 6, the impedances in a high frequency range show such undesirable undulatory characteristics that the impedance at the output terminal 37 at the 1/2 position of the resistance element is locally minimum and those at the output terminals 37 at the 1/4 and 3/4 positions of the resistance element are locally maximum.